The future is a fascinating subject. When it comes to digital technology, however, the crystal balls and Ouija boards are having a hard time keeping up with the latest developments. When it comes to the next-gen widget, it turns out predictions tend to be running behind what is actually happening. There are so many good candidates in the power grid that it’s hard to pick one. There is, however, one grid element that has been getting a great deal of next-gen interest: energy storage.
More specifically, it’s battery energy storage systems (BESS), which has a rather unique position when it comes to the grid. It’s on the transmission system, distribution network, and behind-the-meter. It’s also a critical element for making wind and solar more grid friendly. BESS are considered by some as renewable resource. So, there is a lot of speculation as to what can be expected for the next-gen BESS.
Without getting too technical, let’s say these devices have one thing in common. They produce direct current electricity that needs to be converted into alternating current electricity for use on the power grid. That is done by using inverter-based technology, which is becoming a problem as their numbers increase.
There is a problem, however, these inverter-based devices are replacing massive rotating machines (e.g., generators and turbines). As a result, the grid is losing the inertia these generators produce. Inertia is a form of kinetic energy storage that comes from large synchronous (i.e., the rotation of the shaft is matched with the grid frequency) machines. This kinetic energy provides short term balance between the supply and the load. Before getting into that discussion, let’s dig a little deeper into energy storage in general.
T&D World produced its first energy storage supplement in 2009. The technology was mostly pumped-storage and lead-acid batteries, and there was a lot of activity around renewables. When the second energy supplement came along almost a decade later, the changes were amazing. The emphasis had switched with energy storage becoming an integral part of more sophisticated applications. That’s when BESS became part of distributed energy resources. It was a gamechanger and gave a hint of things to come.
By incorporating BESS technology on both sides of the meter, suppliers tapped into grid stabilization services. Grid resilience started trending, which opened new opportunities for BESS technology. It moved from a niche player to a major force on the power grid.
Interestingly, the 2009 supplement quoted a research organization predicting the global energy storage market would grow from around US$ 329 million in 2008 to more than US$ 4.1 billion by 2018. Well, it did! It actually grew beyond that prediction to about US$145 billion in 2018 according to a recent Fortune Business report. The report also expected the global market spending to reach US$ 211 billion by the end of 2026.
BESS applications are definitely a growth technology. It will continue based on the push for decarbonization with renewable resources. That brings us back to the concerns with electronics-based, inverter-based renewable resources. Fortunately the technology has been improving and there are advanced inverter applications available to address the issues.
Grid-Following vs Grid-Forming
An insight from the NREL (National Renewable Energy Laboratory) comes in the form of a publication saying, “Today’s electric power systems are rapidly transitioning toward having increasing proportion of generation from nontraditional sources, such as wind and solar power, as well as energy storage devices.” These resources are connected to the grid through grid-following inverters. That’s another clue for where the next-gen energy storage is headed along with wind and solar renewables.
This is a good place to talk about inverter technologies. Grid-following inverters track the voltage angle of the grid to control their output. These grid-following inverters rely on the fact that the system voltage and frequency are stabilized by inertia (i.e., rotating masses) sources. BESS using grid-following inverters don’t handle large grid disturbances well. They typically shut down until the disturbance has passed, and require the grid to reestablish after a blackout before reestablishing themselves.
As more large fossil-fueled generating plants are retired and replaced by renewables, the grid needs more stabilizing inertia sources. That is where grid-forming technology comes into play. Grid-forming inverter technology can establish grids and strengthen operating grids. It has an independent internal frequency reference, which allows grid-form inverters to form an island grid.
When grid-forming technology is paired with advanced automation and controls, it makes possible virtual synchronous machines. They can be used to provide services large grids need to operate with lots of renewables. This technological innovation is why grid-forming inverters are starting to generate interest and gain traction for next-gen status.
John Glassmire, senior advisor for Hitachi Energy’s Grid Edge Solutions, provided some actual experience with grid-forming technology including virtual synchronous machines in the real world on the Australian transmission grid. Glassmire reports, “Most battery energy storage deployed globally offers partial network stability, but the next generation of battery energy storage – particularly energy storage that uses grid-forming energy storage with virtual synchronous machine technology – is critical for enabling renewables to fully displace fossil-based synchronous technology.”
Glassmire continued, “For example, Australia pulls 24% of its electricity from renewables, a huge accomplishment for a country of this size. Australia is continually progressing toward net zero emissions, but the output of these renewables are variable. As the installed wind and solar capacities grow, there is a need for new integration technologies. As part of Australia’s commitment to renewables, Hitachi Energy took part in Australia’s Energy Storage for Commercial Renewable Integration, South Australia (ESCRI-SA) project by providing a large-scale grid-edge solution leveraging microgrid technology.”
According to Glassmire, “Hitachi Energy supplied a 30MW BESS on the lower end of the Yorke Peninsula in 2018 on a long radial feeder. The ESCRI-SA BESS is a grid-forming system built on Hitachi’s virtual synchronous generator platform, which strengthens the grid by providing inertia, high fault current, and fast power injection, as well as competitive market services. The Hitachi system is also capable of seamlessly transitioning into island operation when faults occur on the upstream feeder. The island power supply comes from the nearby 91 MW Wattle Point windfarm and distributed solar.”
Before leaving the ESCRI-SA project, Glassmire pointed out, “The virtual synchronous machine offers an extremely valuable service to the grid. It mimics the behavior of old school technologies like synchronous machines and synchronous condensers, but entirely through power electronics. They can even mimic more sophisticated and newer devices like a STATCOM that stabilize grids with benefit of also providing energy and ancillary services. A BESS with a grid-forming inverter including a virtual synchronous machine is a different animal from one with a grid-following inverter system. The automation and controls are a key element to using grid-forming inverters in large utility grids”
Late last year, another grid-forming project was announced in Australia. The Australian utility AGL broke ground on the Torrens Island 250MW/250MWh grid-forming BESS project in November 2021. The battery will be supplied by Wärtsilä with over 100 grid-form inverters supplied by SMA. AGL expects the battery to be fully operational in early 2023. AGL said the BESS is designed to be increased to 1,000MWh in the future. They expect the BESS to take part in Australia’s National Electricity Market.
With the expanding interest in grid-forming technology the U.S. Department of Energy (DOE) announced it is providing funding for the US$25 million public-private Universal Interoperability for Grid-Forming Inverters (UNIFI) Consortium. DOE said, “The Consortium brings together leading researchers, industry stakeholders, utilities, and system operators to advance grid-forming technologies.” The Consortium will be led by NREL (National Renewable Energy Laboratory), EPRI (Electric Power Research Institute), and the University of Washington.
One of major tasks of the Consortium is the development of standards for the hardware needed for these next-gen inverter-based technologies. Interoperability is a key concern for any emerging technology. We have seen in the past that new technologies must play well with each other if they are to be accepted by the power delivery system. That is the reason so many manufacturers such as, Danfoss, Eaton, General Electric, Hitachi Energy, Schneider Electric, Siemens Energy, SMA, and others are interested in the UNIFI project.
The next-gen of inverter-based BESS is here and just in time considering what’s happening. The penetration of wind, solar, and BESS resources is increasing and causing concern by those responsible for a stable grid. When these three resources exceed 60% or more of the online generation capacity operators get nervous. The old-school inverter-based technology can’t provide the inertia needed to generation and load stability of massive rotating machines.
Grid-forming inverters, however, are available to address these concerns, which are happening more often than might be expected. It’s all about adding virtual inertia in a world where clean energy replaces fossil-fuel massive rotating machines. In this case, the next-gen grid-forming BESS is here today!